Tapping the Tab key once in Photoshop will hide the Toolbar and panels (fig 1. Screen space). Tapping it a second time brings s them back. Shift plus the Tab key will hide only the panels. With the panels hidden, placing the cursor at the edge of the monitor (or application frame on Mac) will display the panels enabling access to the tools and panels that you may need to use, when you move your cursor away, they are again hidden (in a sort of roll-over effect).
To toggle off this feature, choose Preferences - Workspace - Auto-Show Hidden Panels or Control -click (Mac) | Right -click (Win) on the name tab of a panel that expanded and check the, Auto-Show Hidden Panels.(fig 2. Screen space)